A coupled atmosphere-ocean-carbon cycle model is used to examine the long term climate implications of various 2050 greenhouse gas emission reduction targets. All emission targets considered with less than 60% global reduction by 2050 break the 2.0 degrees C threshold warming this century, a number that some have argued represents an upper bound on manageable climate warming. Even when emissions are stabilized at 90% below present levels at 2050, this 2.0 degrees C threshold is eventually broken. Our results suggest that if a 2.0 degrees C warming is to be avoided, direct CO2 capture from the air, together with subsequent sequestration, would eventually have to be introduced in addition to sustained 90% global carbon emissions reductions by 2050.